Method for improving knit line strength in polymeric materials

ABSTRACT

The knit line strength and other packing related properties both aesthetic and functional of injection molded polymeric products is improved by equipping the mold with packing pins. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. The packing pins are sequentially actuated during or after filling of the mold to cause flow of plastic either transverse or parallel to the direction of flow in a to and from manner. The result is a kneading action in the viscous flow and an oriented layering effect in the solidified plastic much like a scarf joint at the knit line location. The irregular shape of the layers enhances the mechanical strength at the knit line in addition to increasing the likelihood that long chain molecules will re-entangle between the two melt flows at the knit line. The packing of parts both with and without knit lines can be optimized in localized areas requiring more packing such as thick sections or in areas adjacent to insert molding locator pins which are pulled out of mold cavities during cooling causing voids which must be filled. Packing pins can also be used to cure gateblush by retracting the blush area adjacent the gate during fill and packing it forward into proper part location after the packing is complete. Packing pins can also be used in concert with gates both into and out of a molded part to enhance venting, provide gate shutoff and localize packing at the end of fill in the gate area.

This application is a divisional application from application Ser. No.08/560,327, filed Nov. 17, 1995, now U.S. Pat. No. 5,766,654, issuedJun. 16, 1998, in turn a divisional application from application Ser.No. 08/198,522, filed Feb. 18, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The field of the invention pertains to the injection molding of plasticmaterials and, in particular, to means and methods of improving thestrength of plastic to plastic weld lines or knit lines in moldedproducts and improving the ability to pack out various areas of a moldedcavity. The invention also addresses methodology for improved venting ofmolds and the improvement of gateblush in molded parts.

Often in the injection molding manufacture of plastic products multiplegates or multiple flow paths or both are required in the mold. Welds orknit lines occur in the plastic where the flow paths join. Because ofcertain mold or product designs and injection molding techniques a“cold” weld or knit line can occur because the plastic does not fusetogether completely. The long chain polymeric molecules from each flowfront at the knit line fail to fully re-entangle and regain theintegrity of the solidified melt. The result is a very weak area in theplastic product. Many polymeric materials, especially filled materialsand liquid crystal polymers, exhibit an extreme loss of knit linestrength relative to the balance of the product. Thus, knit linestrength failure is a major cause of production rejects and thestructural failures of plastic products in service.

U.S. Pat. No. 2,191,703 discloses means and method for the eliminationof knit lines and welds by accurately controlling the location of theknit lines and by providing cavities in communication with the moldcavity at the knit line locations. The continued application ofinjection pressure forces the newly formed knit lines out of the moldand into the cavities provided. The result is a product substantiallydevoid of weak knit lines. Unfortunately, after ejection from the moldthe product must be trimmed from the plastic formed in the cavities andthen the trim line polished if needed for appearance.

U.S. Pat. No. 2,199,144 discloses the use of tangential injection abouta toroidal shape such as a steering wheel thereby creating a continuousflow about the mold and a smooth merger of flows at each tangentialgate.

Both of the above disclosures are directed to toroidal shapes withtoroidal shaped strengthening inserts. These approaches to solving theweld or knit line problem are only applicable to a limited number ofgeometries and not generally applicable.

U.S. Pat. No. 4,399,093 discloses the use of differing cooling rates inthe two flows of the molten plastic resin that merge at the knit line.Initially the resin flows fill the mold at relatively low pressure. Uponfilling the mold cavity a high packing pressure is applied causing thecolder less viscous resin to protrude into the warmer more viscous resinat the knit line. The result is a knit line of greater surface area andinterlocking shape to improve the strength at the knit line. However,the differing temperatures of the resin flows are dependent on productshape and cooling rates within the mold and, therefor, limited toproduct shapes and molds that can produce the differential cooling ratesin the resin flows without affecting product quality.

With a view toward providing a method of injection molding that haswider applicability in improving the knit line strength of plasticinjection molded products, applicant has developed the new method andapparatus disclosed below.

SUMMARY OF THE INVENTION

The invention contemplates the use of packing pins in the mold which inconstruction are similar to the ejector pins in most conventionalplastic injection molds. The new packing pins are sequentially actuatedduring or after filling of the mold or during both phases of theprocess. The actuation of the packing pins is normally continued duringcooling as necessary to cause flow of plastic parallel or transverse tothe original direction of flow. (The primary flow causing the knit lineto be strong will be the flow parallel rather than transverse.) The toand from kneading action, especially internal to the frozen plasticskin, results in the equivalent of a scarf joint at the knit line. Thelayering effect perpendicular to the knit line greatly expands thesurface layer contact between the two flows. The irregular shape of thelayers enhances the mechanical strength of the joint in addition toincreasing the likelihood that long chain molecules will re-entanglebetween the two melt flows. Further, the anisotropic characteristics inthe direction of flow that occur with some plastics can be improved bythe scarfing and layering at the knit line.

Other applications of the packing pin concepts are in areas ofrelatively small cross-section or other hard to pack areas in moldedparts. The actuation of packing pins in a localized area or any numberof areas can be either in concert using a common mechanism such as anejector plate or separately using air or hydraulic cylinders, cams orelectrical devices. With separate actuation, each area can be packed adifferent amount.

Packing amounts or degrees can also be adjusted or modified by adjustingthe packing pin diameter or stroke or both depending on the application.

The are other areas of packing pin applications in insert molding wherelocator pins for insert location protrude into the cavity of the mold.These pins are retracted at a proper time, generally after filling. Thefunction of the pins are to support and locate the insert in the cavitywhile the plastic flows around it. After the part is full, the pins areretracted at a proper time which is long enough to ensure that theinsert stays at its desired location, but soon enough to allow plasticto flow into the void created by the retracting pin. Many times the pinsmust be pulled prematurely to allow flow into the void area. Thisresults in movement of the insert and a resultant substandard or rejectpart. Using a packing pin in concert with a locator pin can eliminate orreduce the movement by providing additional localized material anddisplacing it into the locator pin void as or after the locator pin ispulled allowing the void to be filled with the displaced material.

The packing pin concept can also be used in a gate area to reducegateblush caused by plastic entering the mold. Gateblush is caused bydisplacement movement of plastic and impinging on the cavity wall thuseroding the partially solidified plastic on the cavity surface. Apacking pin can be used to retract the cavity wall from the mold surfacein the blush area. After the mold filling is complete, the packing pincan return the cavity surface to the proper position. During fill, thematerial in the “well” area caused by the retracted packing pin will notbe eroded and thus the blush area on its surface will not be present. Inthis situation, when a packing pin is pushed forward excess plastic maybe trapped in the cavity if the gate has been sealed. Excess materialcan be allowed to discharge out of the gate and back into the sprue orrunner of the mold if gate seal has not been effected. The method used(gate seal or non gate seal) depends on the application.

A packing pin can also be utilized in conjunction with a subgate orother type of gate allowing plastic to enter or leave a cavity and thusprovide a valve gating action coincident with packing a part.

In the case of a vent exiting a part, the packing can be used as a“powered vent” shut off within the exit gate runner to enhance theability to remove air and other gases from the mold cavity. The pin canbe in a fully retracted position allowing generous venting action of themold. In an intermediate position the pin will shut off the vent andstill be retracted below the mold's surface and available for the finalpacking. The pin can then be activated to the final flush positionwithin the cavity to complete the packing process as previouslydescribed.

Pins which are only used to shut off vents as described above and notused for additional packing are also considered part of this invention.

A tapered egress from the vent pin to the outside of the mold surface isneeded to allow the clearing of the vent and pin should plastic materialbe injected through the vent pin area while the pin is in the fullyretracted position. This tapered egress can progress out to the side ofthe mold or can be angled to intersect the mold parting line whereventing can be done in a conventional manner.

Packing pins can also be used in concert with the gates allowingmaterial to enter into the part. This would be especially advantageousin subgates into the pin well. The activation of the packing pin willcoincidentally seal the gate and allow additional packing using the pin.This will then allow shorter holding times and allow for more time tomelt plastic for the next shot.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the packing action of the packing pinsin communication with the mold cavity and in contact with the coolingplastic resin;

FIG. 2 illustrates schematically in perspective the packing action ofthe packing pins and packing wells in communication with the mold cavityand in contact with the cooling plastic resin;

FIG. 3 illustrates schematically the combination of packing wells andpacking pins with the sprue or runner system of a hot runner mold;

FIG. 4 illustrates schematically a mold insert positioned by locatorpins;

FIG. 5 illustrates schematically a packing pin and mold ventcombination;

FIG. 6 illustrates schematically a packing pin and parting line moldvent combination;

FIG. 7 illustrates schematically a packing pin and sub gate combination;and

FIGS. 8A and 8B illustrate schematically a packing pin solution to gateblush erosion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a portion of a mold cavity 10 is shown filled with viscousplastic resin 12 in the process of cooling. The location of the knitline or weld line is indicated by the dashed line at 14. This portion ofthe mold cavity 10 and plastic product being formed is a flat plate inform. Completely surrounding the mold cavity 10 is the mold body 16.

In communication with the mold cavity 10 are a pair of cylindrical holes18 and 20 having a pair of packing pins 22 and 24 located therein. Thepacking pins 22 and 24 are slidable tight in their respective holes 18and 20. The packing pins are suitably driven by a pair of hydraulic orpneumatic pistons 26 and 28 shown schematically. At the top of eachpacking pin stroke the tops of the packing pins 30 and 32 are level withthe bottom 34 of the mold cavity 10.

With filling of the mold cavity 10 with resin 12 the cylindrical hole 18containing retracted packing pin 22 also fills at 36 with resin. Thepacking pins 22 and 24 may be actuated to repeatedly stroke during orafter filling with resin or both during and after filling with resin. Asthe pins 22 and 24 move to displace the plastic in the volume 36 andsimilar volume in hole 20, the knit line 14 becomes rippled (as shown bythe solid line 38) in response to the to and fro motion in combinationwith the skin forming and cooling of the resin 12 in the mold cavity 10.Depending on overall mold cavity shape packing pins 22 and 24 to eitherside of the knit line 14 may be linked 40 together to providealternating strokes. However, the final pack preferably moves bothpacking pins 22 and 24 to the top of their respective strokes with thetops 30 and 32 even with the bottom 34 of the mold cavity 10. This canbe accomplished by moving the fulcrum 42 upward as indicated by thearrow 44.

In FIG. 2 the resin 12 within the mold cavity is illustrated inperspective with the mold deleted. The resin 12 is supplied to the moldcavity through a double sprue 46 and gates 48. Knit lines occur at threelocations 50, 52 and 54. Below and to either side of knit line 54 is apair of packing pins 22 and 24 as detailed in FIG. 1 and operated asexplained for FIG. 1. Although similar pairs of packing pins might beemployed for knit lines 50 and 52, a modified construction isillustrated. The modified construction comprises wells 56 and 58 toeither side of the mold cavity opposite knit line 52 and wells 60 and 62opposite knit line 50. The wells 56 and 58, 60 and 62 as shown are incommunication through gates 64 with the mold cavity and filled withresin 12 as the mold is filled. As is illustrated further below in FIG.3 the wells 56, 58, 60 or 62 can be combined with a hot runner orinsulated runner system used to supply the material from the injectionunit. The packing pins are used to provide packing of the mold cavity aswell as providing the scarf joint action at the knit lines.

Extending below each well 56, 58, 60 and 62 are cylindrical holes withthe packing pins 66 shown. The reciprocating movement of the packingpins 66 causes flow of resin 12 in and out of the mold cavity asindicated by arrows 68. The packing pins 66 operate generally in thesame manner as in FIG. 1, however, this configuration, although morecomplicated in mold design, eliminates surface blemishes on theunderside of the resin product. Use of the wells also permits the use ofpacking pins where the product thickness is not sufficient to directlyapply the pins at the locations of the gates 64 or for other reasonsrelated to mold or injection machine configurations.

Illustrated in FIG. 3 is a mold 70 having a cavity 72. To supply hotmolten resin to the cavity 72 there is a sprue 74 from the injector (notshown) and a pair of runners 76 and 78 leading to a pair of wells 80 and82 with gates 84 and 86 communicating with the cavity. Fitted in thewells 80 and 82 are a pair of packing pins 88 and 90 shown in retractedposition. Each packing pin is activated by a hydraulic, pneumatic orother gas powered piston (nitrogen or other inert gas) and cylindermechanism as shown at 92 and 94. The sprue 74, runners 76 and 78, wells80 and 82, and gates 84 and 86 are formed by a manifold 96 separatedfrom the mold 70 by an air gap 98. Thus, the manifold forms a hot orinsulated runner system. After the cavity 72 and the wells 80 and 82 arecompletely filled with resin both packing pins 88 and 90 can moveforward an amount sufficient to seal off the wells from the runners 76and 78. Then the packing pins 88 and 90 can be alternatingly extendedand retracted to form the knit line 100 into a scarf-like joint.Although only two combined wells and packing pins are shown in FIG. 3 aplurality can be positioned to either side of the knit line 100 as inFIG. 2. Thus, sets of packing pins may operate in tandem.

In one example of the method of using the packing pins, the sequencewould start with one set of packing pins retracted and the other set ofpacking pins at the top of their stroke. With the start of resininjection the mold cavity would be filled quickly, filling the volumesabove the retracted pins. With the mold cavity filled and pressurizedwith resin, the packing pin sets can be reciprocated causing theretracted pin sets to move toward the mold cavity and the other pin setsto move away from the mold cavity. As a result the resin is displacedacross the direction of flow in a substantially linear fashion betweenthe locations of oppositely moving packing pins. The resin not alreadysolidified near the mold cavity walls flows, and with the cooling andfreezing, the orientation effect above occurs at knit lines betweenoppositely moving packing pins. The sequencing of the packing pins maybe repeated at various intervals or only once depending upon theorientation effect required for the application.

Both sets of pins can start the injection cycle retracted or partiallyretracted to allow additional material to be put in the mold. This willallow more resin for more packing than with only one set of packing pinsretracted. Both sets of packing pins can be sequenced forward initiallyin the packing sequence and then the reciprocating sequence may commenceif appropriate. Referring to FIG. 3, by activating both packing pins 88and 90 at the end of the injection stroke, the packing pins inject hotresin from the manifold into the mold cavity 72 to provide a finalpacking in the injection step. This technique minimizes the staticpressure losses in the sprue 74, runners 76 and 78 and wells 80 and 82and results in greater process uniformity and less product variability.

As a further alternative the method may also be initiated using only onepin on only one side of the knit line. Packing can be initiated bypushing the pin forward and the scarf-joint like flow can beaccomplished by the additional internal compression of the unsolidifiedresin which will reverse the flow by expansion when the pin isretracted.

After the reciprocating sequencing is complete the retracted packingpins may be extended to their top most position to do a final packing ofthe resin in the mold cavity. The final stroke is adjusted to normallydisplace the resin necessary to make up for some or all of the shrinkageresulting from cooling and freezing of the resin. Thus, these packingpins are capable of additional packing during the solidificationprocess, minimizing shrinks and voids in the plastic products.

The disclosure above is quite general with respect to the placement ofthe packing pins and mechanisms for operating the packing pins. Thepacking pins may be actuated mechanically, hydraulically, pneumaticallyor electrically as with ejector pins depending on the particularapplication. Activating the packing pins pneumatically or hydraulicallyimparts a hydrostatic force to the resin in the mold cavity thatultimately results in a uniform pressure across the resin in the moldcavity and uniform part shrinkage. Packing velocity can be controlled bycontrolling actuator speed by conventional means. Packing velocity iscritical in some applications when repeatable stresses and staticpressure gradients are important to provide repeatable partcharacteristics. The final stroke or pack need not bring the tops of thepacking pins flush with the wall of the mold cavity but rather someresin protrusions may be allowed to extend into the packing pin holes.Thus, the final stroke may be determined by pressure in the resin in themold cavity rather than mechanical position.

Depending on mold configuration the packing pins may be incorporated inmovable or stationary portions of the mold. As disclosed above thepacking pins may directly communicate with the mold cavity orcommunicate through wells and gates or through cold, hot or insulatedrunner systems to the mold cavity. Moreover, the packing pin system maybe used to affect the physical properties of the cooling resinregardless of whether a knit line is present.

In FIG. 4 an insert 102 is positioned and held within the mold cavity104 by a pair of moveable locator pins 106 and 108. The locator pins 106and 108 are normally withdrawn as sufficient resin is injected into thecavity 104 to retain the insert in place. In this embodiment, a pair ofmoveable packing pins 110 and 112 are located next to the locator pins106 and 108. The packing pins 100 and 112 are retracted as shown, but,as the mold cavity 104 fills with resin and the locator pins arewithdrawn, the packing pins 110 and 112 are activated to push resin intothe voids formed by the withdrawing locator pins. To simplifyconstruction, each of the pins is half round in cross-section, thus theupper location pin 106 and packing pin 110 fit within a round hole 114in the upper half of the mold. Likewise, the lower locator pin 108 andpacking pin 112 fit within a round hole 116 in the lower half of themold.

FIG. 5 illustrates a packing pin 118 retracted below a vent 120 thatopens through the mold wall 122. The packing pin hole 124 communicateswith the mold cavity 126 at a location spaced from the runner 128 andgate 130 to the mold cavity. As the mold cavity 126 fills with resin,air from the cavity leaves through the vent 120. At an appropriatemoment in the injection cycle, as the resin moves into the packing pinhole 124, the packing pin 118 moves to the intermediate position 132wherein the vent 120 is closed to prevent resin from flowing out thevent. The packing pin can then be subsequently moved to the cavity wallor reciprocated as noted above.

In FIG. 6 the vent 120 is further modified by adding a vent well 134along the parting line 136 of the mold 122. The vent well 134 is incommunication with an ejector pin hole 138 and pin 140 such that anyresin that has flowed out the vent 120 and into the well 134 can beejected by the ejector pin 140 as the mold opens to release the resinproduct. The vent well 134 communicates with a conventional mold vent135 to allow gasses an egress out of the mold.

FIG. 7 illustrates further the use of the packing pin to close anindirect or subgate to the mold cavity. As shown, the runner 142supplies a subgate 144 that communicates with the mold cavity 146through a packing pin hole 148. Once the mold cavity 146 is filled, thepacking pin 150 first closes the gate as shown by 152 and then movesfurther toward the cavity to complete the packing of the resin. Uponsolidification of the resin, the mold parts along the parting line 154to eject the product and the solidified resin in the runner 142 is alsoejected by a pin 156. The gate 144 in FIG. 7, as well as the vents inFIGS. 5 and 6, are tapered to provide easy removal of resin trapped andsolidified therein.

In FIGS. 8A and 8B the packing pin solution to gateblush is illustrated.Gateblush occurs when the hardening skin of resin 158 within the moldcavity 160 is eroded by the flow of additional resin shown by arrows 162impinging upon the mold wall as at 164. To overcome this damage, whichshows up as surface discoloration and streaking, the affected portion ofmold wall is retracted by a packing pin 166 forming a part of the wall.At about the instant the injection of resin is completed, the packingpin is extended flush with the wall to pack the resin in the cavity. Theresin 168 that collected on the retracted pin has not been eroded thuspreventing gateblush on the finished product.

I claim:
 1. A method to improve certain characteristics of injectionmolded plastic products comprising injecting molten plastic resinthrough at least one gate into a mold cavity within a mold body, saidmold body being equipped with at least one non-injection well andnon-injection well gate in the mold body, said non-injection well gateof a cross-section substantially less than the cross-section of thenon-injection well, said non-injection well in communication with themold cavity only through the non-injection well gate and at least onereciprocateable packing pin penetrating the mold body into communicationwith the non-injection well, non-injection well gate and the moldcavity, and subjecting at least a portion of the molten plastic resin inthe mold cavity and non-injection well to a to and fro movement byreciprocating the packing pin as the molten plastic resin solidifies. 2.The method of claim 1 wherein the to and fro movement of the moltenplastic resin is initiated subsequent to the filling of the mold cavitywith resin.
 3. The method of claim 1 wherein upon the ceasing of the toand fro movement, an excess amount of molten plastic resin is packedinto the mold cavity to fill shrinkages and voids.
 4. The method ofclaim 1 wherein the reciprocating movement of the packing pins causingthe to and fro movement is ceased and all packing pins are caused toreach and retain their top most positions prior to opening the mold inthe molding cycle.